Slips are used to hold casing in position while additional lengths of casing are being added to the casing string. Known slips include gripping surfaces which significantly damage the surface of the casing.
A gripping surface for use in tongs comprising a resilient base member embedded on its surface with particles of carborundum is also known. However, while this gripping surface causes minimal damage to the surface of the pipe being gripped, it is insufficient to safely retain long casing strings.
The present invention aims, at least in its preferred embodiments, to provide an insert which will adequately grip reasonably long casing strings with minimal damage to the casing.
According to the present invention there is provided an insert for use in slips, which insert comprises a resilient base member which supports a plurality of strips having a pipe gripping surface.
Preferably, said pipe gripping surface comprises teeth. However, it could also comprise particulate material bonded to said strips. Such material could comprise, for example carborundum or diamond grit.
Advantageously, said teeth have a leading face which is inclined at an angle .alpha. of from 60.degree. to 120.degree. to the longitudinal axis of the strip.
Preferably, a is substantially 90.degree..
Advantageously, said teeth have a trailing face which is inclined at an angle .beta. of from 20.degree. to 50.degree. to the longitudinal axis of the strip.
Preferably, .beta. is substantially 35.degree..
Advantageously, the depth of said teeth is from 0.2 mm to 2 mm.
Preferably, d is substantially 1 mm.
If desired, the teeth may be of differing hardness, for example relatively hard teeth towards the top of the insert and relatively soft teeth towards the bottom.
Advantageously, said strips, in use, extend substantially parallel with the longitudinal axis of casing to be gripped. If desired, said strip could be inclined to the longitudinal axis of the casing. In an alternative embodiment, the strips would be arranged perpendicular to the longitudinal axis of the casing to be gripped. Such an embodiment could be used where resistance to torsional forces is particularly important.
Preferably, said strips project beyond said resilient base member to accommodate debris from the surface of said casing. This arrangement is particularly useful since it allows space between the strips to accommodate paint and other debris derived from the surface of the casing. However, it would also be possible for the surface of the strips to be flush with the surface of the resilient base member.
Advantageously, said pipe gripping surface is covered with a material which is corrosion-resistant, for example titanium.
The present invention also provides an adaptor provided with a plurality of inserts in accordance with the invention.
If desired, each insert may have teeth of a specific hardness, with the hardness of the teeth of one insert differing from the hardness of the teeth of another insert.
There is also provided slips provided with one or more adaptors in accordance with the present invention.
The present invention also provides an elevator incorporating an insert in accordance with the present invention.